The present invention relates to the detection of faults in the rotor windings of large, synchronous dynamoelectric machines, such as turbine generators.
Large turbine generators normally have either two or four poles, and the field winding is a distributed winding made up of copper conductors placed in slots on a cylindrical rotor. The method and apparatus herein disclosed, however, can be applied equally well to a rotor having any number of salient poles with concentrated windings. Faults may occasionally occur in either type of such rotors, usually either shorted turns or faults to ground, it is, of course, desirable to be able to detect the occurrence of such faults promptly. In many cases, these machines have collector rings on the rotor for supplying field excitation to the rotor winding, and the rotor circuits are thus easily accessible during operation and faults can readily be detected by simple measurements. This cannot be done, however, in machines with brushless excitation in which the field excitation is supplied through a rotating rectifier assembly from an alternating current exciter with its armature winding on the rotor, so that the exciter armature, the rectifier assembly and the main generator field winding all rotate together on a common shaft. This type of excitation system has many advantages and is widely used, but the rotor circuits are not accessible during operation of the machine so that the occurrence of a fault in the rotor windings cannot be easily detected by simple resistance or similar measurements.
Faults in the rotor winding result in asymmetries in the winding which cause even harmonics to occur in the flux wave produced by the winding. Second or other even harmonics, therefore, appear in the generator voltage and can be detected in the line-to-neutral voltage, and it has been proposed to detect the occurrence of faults in the rotor winding by monitoring the generator voltage for even harmonics. This has the disadvantage that it is not usable with a full-pitch generator winding because the even space harmonics would be completely canceled out. Even with the usual winding of less than full pitch, the even harmonics largely canceled so that the harmonic voltage to be detected is a very small component of a relatively large voltage and very sensitive measuring devices are required. Furthermore, some types of loads, such as rectifiers, result in second harmonics in the generator winding due to the non-linear type of load and such harmonics cannot be distinguished from those due to rotor faults. For these reasons, monitoring the harmonic content of the line-to-neutral generator voltage is not a desirable method for detecting rotor faults. The presence of ground faults in the rotor winding has been detected by directly measuring the resulting ground current by suitable auxiliary means, as in Hoover et al U.S. Pat. No. 3,303,410 and Williamson et al U.S. Pat. No. 3,593,123, while the occurrence of shorted turns in the rotor winding may be detected in the manner suggested in Albright et al U.S. Pat. No. 3,506,914 by observing differences in amplitude of the slot leakage flux in the air gap.